Degeneration of a joint such as a spinal segment by a deterioration of the hard and soft tissues of the joint complex may produce severe local or radiating pain when that segment is in motion. Typically joint complexes consist of two bony structures and an interposed flexible, movable portion. In the spine the bony structures are the vertebrae and the movable portion is the intervertebral disc. The disc is composed of a multilayered outer ligamentous belt, the annulus, constructed in concentric laminations rather like the plies of an automobile tire. In the core of the disc there is a small mass of flexible fibrogel, contained by the annulus ring. The fibrogel mass, the nucleus of the disc, is a hydrogel which on absorbing water exerts a substantial swelling pressure to lift the vertebra and balance the forces applied against the disc by gravity and surrounding muscular contractions. Therefore, the hydrogel is important for resisting potentially disruptive forces applied to the vertebrae.
Unfortunately, as the disc degenerates, the internally contained hydrogel begins to lose its water-binding ability and shrinks. This shrinkage leads to a loosening of the annulus fibers which permits an abnormal range of motion of the segment with buckling and delamination of the overlapping plies. Tears in as few as several layers of the approximately 12 to 20 concentric laminations of the annulus may permit a herniation of the pressured central nucleus material outward through the annulus defect.
Conventional procedures for treating degenerative vertebral discs involve fusing the discs together to stop all motion of the bone segments. The most efficient method of fusion places bone or a bone inducing substance inside a supporting device surgically implanted into the center of the disc. This supporting device construct will obliterate the degenerated nucleus, hold the bone material rigidly in position, protect the bone from collapse, extrusion or invasion by residual soft tissues of the disc and cause the opposing vertebrae to rapidly fuse together. The preferred intervertebral fusion device is a vertebral fusion cage. For example, U.S. Pat. No. 4,961,740 to Ray, contents of which are incorporated herein, discloses threaded vertebral fusion cages. The internal cavities of the cages are used to secure the bone graft material and to permit bone growth through and across the surgically emptied nucleus cavity between adjacent vertebrae.
As opposed to non-threaded fusion cages which are hammered or tapped into position, insertion of threaded vertebral fusion cages is made more efficient because the threaded outer surface permits easy adjustment of the depth and penetration of the cage into the disc space. The threaded outer surface also prevents dislodgment or expulsion of the cage. In addition, the graft bone packed within the threaded fusion cages presents or effuses through these perforations and comes into intimate contact with the bone of the adjacent vertebral bodies. When the cage is inserted into the bored or tapped intervertebral bed, the lateral walls of the cage are oriented horizontally and face the disc cavity. These lateral cage walls are blocked (i.e., contains no apertures) and therefore are a barrier against any potential ingrowth of residual disc tissue into the contained graft area which could interfere with or weaken the fusion formation of these adjacent vertebrae.
More recently, emphasis has been placed on securely fixating the fusion cage implant within the vertebrae. During a fusion cage implantation procedure, the surgeon may determine that sufficient stabilization of the space has not been achieved by implantation of the fusion cage alone. In such situations, additional instrumentation to improve the stability of the vertebrae and cage is required. Examples where additional stabilization procedures may be used include: the vertebral bone is weak, the cages do not fit tight enough in the vertebral space or the central concavity of the disc space is too deep to achieve good cage penetration along the anterior-posterior length of the disc space. In such cases, the surgeon ordinarily would be forced to place additional fusion instrumentation such as pedicle screws, rods or vertebral body plates to prevent cage dislodgment and improve the opportunity for a good fusion. This additional step in the surgical procedure increases the complexity, potential hazards and cost of the procedure. The embodiments of the present disclosure solve these and other associated problems and provides a simple and easily applied instrumentation to intraoperatively achieve increased cage fixation and disc space stability.